In oil and gas drilling operations there are various techniques for inserting or removing tubing or pipes or lines into or from wells. Well intervention techniques include the use of coiled tubing, wirelines or a process where individual pipe sections are screwed together to form a continuous pipe in the well, also called snubbing. In a coiled tubing intervention, a continuous tube initially wound on a spool is unwound, straightened and pushed down a hole. In order to do this various items of equipment are located in the area above the well, all supported on a common frame. A typical set-up involves, from top to bottom, a goose neck for guiding and straightening the coiled tubing as it is fed from the spool, an injector head for pushing the tubing into or pulling it out of the well, a stuffing box to form a seal around the tubing, and a blow out preventer. There may be additional components depending on specifics of the operation performed. The blow out preventer itself may consist of a number of components, such as a pipe ram, a slip ram, a shear ram and a blind ram.
Regarding coiled tubing well intervention being performed from a floating vessel in a sub sea well, before the coiled tubing can be fed into the well, it is necessary to prepare or rig up all the equipment on top of a riser, which is attached to the top of the well at the sea floor. The riser extends from the top of the well at the sea floor, up to the work-deck of the vessel. The intervention equipment is then rigged up in a frame, which supports the attachment of the intervention equipment to the riser. On floating drilling or well intervention vessels such as semi-submersible rigs, mono-hull vessels or similar, the frame structure also provides the necessary rigidity of the rig-up in order to enable the onboard heave compensating system to function efficiently. The heave compensating system enables the equipment attached to the well to move relative to the floating vessel in a controlled manner. The frame is usually installed by attaching the top of the frame to a hoist suspended from a tower above the well and then hoisting the frame into position above the well while transferring the frame from a horizontal to a vertical position above the well. The weight of the frame is carried by the hoist and tuggers are used to stabilise the frame laterally in position above the well. The various equipment items are then attached to the frame, whilst it is suspended above the well. During this rigging up sequence, other activity in the area above the well has to cease. Typically, the sequence can take 12 to 30 hours, or even longer when weather conditions are bad.
In the case of a wireline intervention a frame is similarly used to support equipment including a stuffing box, lubricator and a blow out preventer. A snubbing intervention, involving the use of pipe sections joined together rather than a continuous tubing or line, requires a snubbing unit to be supported in the area above the well.
Offshore well interventions are sometimes carried out from semi submersible rigs or ships. During the rigging up sequence, the rig or ship may be heaving due to weather conditions, making the rigging up more difficult, time consuming and hazardous. If the weather conditions and heave are particularly bad, then rigging and other preoperational activity may have to stop until calmer conditions prevail, in order to secure the safety of the operation.